The present invention relates generally to power supply protection circuits and, in particular, to an improved protection circuit for a switching regulator power supply.
Switching regulator power supplies are commonly used for developing a regulated output voltage for operating electronics equipment such as computers, video monitors and the like. Among other desirable features, these power supplies are extremely efficient in terms of power dissipation. In a typical switching regulator power supply, a DC voltage, developed by a battery for example, is coupled to an input of the power supply and converted to a regulated DC output voltage. Regulation is achieved by using a drive electronics circuit to monitor the output voltage level and to develop a pulse width modulated drive signal for controlling the conduction of one or more output transistors so as to adjust the output voltage in a direction to maintain a desired regulated voltage level. A double-ended output circuit may be used comprising a pair of push-pull operated high voltage output transistors responsive to the pulse width modulated drive signal, an output transformer and a rectification circuit. Alternatively, a single-ended output circuit comprising a single output transistor may be employed. In either case, the battery voltage is normally coupled to an input of the drive electronics circuit and also to a tap on the output transformer. The drive electronics circuit is especially susceptible to voltage spikes developed by the battery and must be protected therefrom in some manner. For example, it has recently been proposed to incorporate electronic apparatus such as computers and video monitors in automotive vehicles of the type normally having high capacity storage batteries which can develop signal spikes as high as 140 volts. Signal spikes of this magnitude can easily damage the power supply drive electronics circuit which normally comprises a number of relatively low level electronic devices.
Prior art protection circuits for switching regulator power supplies typically incorporate a zener diode connected to the base electrode of a transistor whose collector-emitter circuit is connected in series between the battery and the drive electronics circuit. The battery voltage is also coupled across the zener diode so that the voltage applied to the base of the series-pass transistor is limited by the zener diode to a selected safe level despite the presence of large signal spikes in the battery voltage.
Prior art protection circuits of the type described above exhibit a number of undesirable characteristics which are largely attributable to the use of the zener diode in the base circuit of the series-pass transistor. Initially, zener diodes are relatively expensive and are therefore not particularly well suited for use in mass-produced consumer articles. In addition, zener diodes are relatively unstable when exposed to wide temperature variations such as would be experienced in the environment of an automotive vehicle. Zener diodes also have a relatively slow response time wherein adequate protection for rapidly occurring signal spikes is not assured.
Another problem characterizing the prior art switching regulator protection circuits involves operation at marginally low battery voltage levels. In particular, due to the voltage drop across the collector-emitter circuit of the series-pass transistor, the voltage coupled to the drive electronics circuit of the switching regulator will always be less than the actual battery voltage. In the case of a marginally low battery voltage, the further voltage reduction introduced by the series-pass transistor may result in a voltage being coupled to the drive electronics circuit which is too low to effect operation thereof.